This invention relates to structures for high frequency fundamental resonators, particularly for frequency sources and for filters. More particularly, the invention relates to ferrite oscillators and filters, such as yttrium-iron-garnet (Y.sub.3 Fe.sub.5 O.sub.12 or YIG) oscillators and filters.
Ferrite resonators are a favored type of resonator for both oscillator and filter applications because of their typically high resonant frequency (about 1 GHz to about 100 GHz), wide tuning range (typically over one octave), linear tuning characteristics, and spectral purity (high Q factor). A ferrite element may be used in a resonator structure in several ways.
A preferred type of ferrite element is a highly polished sphere of single-crystal material. A spherical shape provides a boundary condition that approximates an infinite volume of ferrite material (thus allowing uniform and predictable resonant modes), and a highly polished surface minimizes surface scattering, and improves the resonant quality of the sphere. The sphere is typically placed in a magnetic circuit, such as within a gap between two magnetic pole faces, that applies a magnetic field sufficient to magnetically saturate the sphere and initiate resonance. A loop or loops couple to the sphere and may transfer resonant energy into or out of the sphere.
Some conventional oscillators attach a ferrite sphere to a long, slender mounting rod, which is inserted through the body of the magnetic circuit to hold the sphere in the air gap near the coupling loops. Most rods are made of electrically non-conductive material near the sphere to avoid eddy currents in the rod affecting the electromagnetic field pattern near the sphere. Some rods have been fabricated out of sapphire or alumina. Other rods use a non-conductive tip in an otherwise metal rod. Such tips have been made of beryllia (BeO), in addition to alumina (Al.sub.2 O.sub.3) and sapphire tips. These non-conductive materials are often chosen for their relatively high thermal conductivity.
The resonant frequency of a ferrite element may drift with temperature. The amount of frequency drift of a YIG sphere over temperature, for example, depends on the crystallographic orientation of the sphere to the applied magnetic field. Certain orientations will exhibit a positive frequency drift with increasing temperature, and others will exhibit a negative frequency drift with increasing temperature. Between these two regions lie thermally-compensated (TC) axes, as is known in the art, where the resonant frequency does not drift with temperature. These TC axes are usually a desirable orientation, hence provision is typically made to rotate the sphere-rod assembly within the resonant structure to align a TC axis with the applied field.
Achieving the ideal, or near ideal, TC solution angle typically involves orienting the sphere along a known axis prior to mounting the sphere on a rod. Orienting the sphere and/or subsequent mounting of the sphere usually requires specialized equipment and knowledge. One crystallographic orientation commonly used for this purpose is to align the sphere so that a [110] axis is normal to the axis of mechanical rotation, and thus a (110) plane may be normal to the applied field.
This alignment can be very difficult to achieve, and some ferrite resonator applications couple significant power to the sphere, thereby heating it. For these reasons, some conventional designs have sought to remove heat from the sphere via its mounting rod to reduce temperature effects. However, providing a heat conduit down the mounting rod may create a thermal gradient across the sphere, altering its uniform resonant characteristics and producing other temperature effects.
Conventional designs typically clamp the rod 101 to the body of the magnetic circuit after aligning a sphere 114, which may be attached to the rod 101 with adhesive 116, in the resonant circuit, as shown in FIG. 1A. This may create a cantilever member supporting the sphere 114, which may be susceptible to mechanical vibration. The cantilevered sphere may move differentially from the loop 115 that is mounted on a substrate 111, and induce a mechanical resonance that appears as phase jumps or frequency breaks offset from the tuned frequency of the structure, as shown in FIG. 1B, or as an instantaneous change in the resonant frequency. Such vibration-induced responses are highly undesirable in both oscillator and filter applications. For example, a vibration-induced phase jump may pull a phase-locked oscillator outside of its phase-locked loop bandwidth, thereby losing the desired output until phase lock is re-established.
One approach to avoid differential motion between a ferrite sphere and its coupling loop is to glue or otherwise fix the sphere under the loop in the magnetic circuit. This approach does not allow in situ TC axis alignment, however, and therefore these structures may exhibit higher thermal drift. Gluing a sphere in a resonator structure also makes it impractical to swap ferrite spheres in and out of the resonator structure. Spheres may have inclusions or other defects that are not apparent until the sphere is placed in the resonant structure, where the defects cause power holes, crossing resonant modes, or other problems. Often, swapping the sphere would destroy the microcircuit and coupling loop.
Therefore, it is desirable to reduce the differential motion between a ferrite sphere and associated coupling structures in a resonant circuit. It is also desirable to accomplish this in a manner that allows ferrite spheres to be exchanged in a resonant structure without damage to other components of the resonant structure, and to provide a resonant structure less sensitive to thermal variations.